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Thread: Human ancestors and chimps may have dated

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    Human ancestors and chimps may have dated

    Did humans and chimps once interbreed?

    Bob Holmes


    IT GOES to the heart of who we are and where we came from. Our human ancestors were still interbreeding with their chimp cousins long after first splitting from the chimpanzee lineage, a genetic study suggests. Early humans and chimps may even have hybridised completely before diverging a second time. If so, some of the earliest fossils of proto-humans might represent an abortive first attempt to diverge from chimps, rather than being our direct ancestors.


    We can observe the traces of this complex history in the human genome today, says David Reich, a population geneticist at the Broad Institute and Harvard University in Cambridge, Massachusetts. Reich and his colleagues compared the genomes of humans, chimps and gorillas using a "molecular clock" to estimate how long ago the three groups diverged. The further back two species diverged, the more differences will have accumulated between their genome sequences.


    The team estimated that humans and chimps diverged no more than 6.3 million years ago, and probably less than 5.4 million years ago, although some parts of the genome showed divergence times up to 4 million years older. Even if we split from our ape relatives 6.3 million years ago, that is still later than some of the earliest fossils showing human-like traits such as altered tooth structure and bipedalism (Nature, DOI: 10.1038/nature04789). "That makes the fossil record even more interesting," says Reich. "What were those fossils?"


    The answer might lie in a second striking observation. Reich's team found that the X chromosome diverged later than any of the other chromosomes. One way this can arise is if natural selection has been acting unusually strongly on genes on the X chromosome. That is significant because in every animal species studied, genes that make hybrids less fertile than their parental species tend to be found on the X chromosome or its equivalent, so hybridisation can create strong selection pressures on this chromosome.
    The best explanation for these surprising findings - the relatively young and variable divergence dates between the human and chimp lineages, and the evidence for strong selection on the X chromosome - would be if the two lineages split sometime before the time of the first proto-human fossils, but later rehybridised (see Diagram) in a "reverse speciation" event (see "When evolution runs backwards"). Natural selection would favour those hybrid individuals whose X chromosomes carried fewest of the genes that lower fertility.


    So far, Reich admits, this is only a plausible hypothesis, not a proven fact. For example, he calibrated his molecular clock using the divergence time between humans and macaques, which is estimated at no more than 20 million years ago. If this divergence happened earlier, that would push back the human-chimp split to an earlier date as well - perhaps far enough that there would be no need to invoke hybridisation. At the very least, though, Reich's study shows that the separation between humans and chimps was a long, drawn-out process.

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  2. #2

    Re: Human ancestors and chimps may have dated

    Human and chimp genomes reveal new twist on origin of species

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    The relative age of genetic changes between the human and chimp genomes varies over a period of ~ 4 million years, depending on where in the genome you look.

    The evolutionary split between human and chimpanzee is much more recent -- and more complicated -- than previously thought, according to a new study by scientists at the Broad Institute of MIT and Harvard and at Harvard Medical School published in the May 17 online edition of Nature. The results show that the two species split no more than 6.3 million years ago and probably less than 5.4 million years ago. Moreover, the speciation process was unusual -- possibly involving an initial split followed by later hybridization before a final separation.

    "The study gave unexpected results about how we separated from our closest relatives, the chimpanzees. We found that the population structure that existed around the time of human-chimpanzee speciation was unlike any modern ape population. Something very unusual happened at the time of speciation", said David Reich, the senior author of the Nature paper, and an associate member of the Broad Institute and assistant professor at Harvard Medical School's Department of Genetics.

    Previous molecular genetic studies have focused on the average genetic difference between human and chimpanzee. By contrast, the new study exploits the information in the complete genome sequence to reveal the variation in evolutionary history across the human genome. In theory, scientists have long known that some genomic regions must be 'older' than others, meaning that they trace back to different times in the common ancestral population that gave rise to both humans and chimps (see Graphic). But, the new study is the first to actually measure the range of ages. It gave three surprising results:
    • the time of from the beginning to the completion of divergence between the two species ranges over more than 4 million years across different parts of the genome. This range is much larger than expected.
    • the youngest regions are unexpectedly recent -- being no more than 6.3 million years old and probably no more than 5.4 million years old. This finding implies that human-chimp speciation itself is far more recent than previously thought.
    • if one looks only at the X chromosome, it almost entirely falls at the lower end of the time frame. In fact, the average age of the X chromosome is ~1.2 million years "younger" than the average across the 22 autosomal (non-sex) chromosomes.

    "The genome analysis revealed big surprises, with major implications for human evolution," said Eric Lander, Director of the Broad Institute and co-author of the Nature paper. "First, human-chimp speciation occurred more recently than previous estimates. Second, the speciation itself occurred in an unusual manner that left a striking impact across chromosome X. The young age of chromosome X is an evolutionary 'smoking gun.'"

    The estimate that humans and chimpanzees probably split less than 5.4 million years ago is more recent by ~1 to 2 million years than a previous estimate of 6.5-7.4 million years based on the famous Toumaï hominid fossil (Sahelanthropus tchadensis), which has features thought to be distinctive to the human lineage.

    "It is possible that the Toumaï fossil is more recent than previously thought," said Nick Patterson, a senior research scientist and statistician at the Broad Institute and first author of the Nature paper. "But if the dating is correct, the Toumaï fossil would precede the human-chimp split. The fact that it has human-like features suggest that human-chimp speciation may have occurred over a long period with episodes of hybridization between the emerging species."

    The possibility of "hybridization" -- that is, initial separation of the two species, followed by interbreeding and then final separation -- would also explain the strange phenomenon seen on chromosome X. Interbreeding is known to place strong selective pressures on sex chromosomes, which could translate to a very young age for chromosome X.

    "Hybridization" is commonly observed to play a role in speciation in plants, but evolutionary biologists do not generally view it as an important way to produce a new species in animals.

    "A hybridization event between human and chimpanzee ancestors could help explain both the wide range of divergence times seen across our genomes, as well as the relatively similar X chromosomes," said Reich. "That such evolutionary events have not been seen more often in animal species may simply be due to the fact that we have not been looking for them."

    As the researchers note in the Nature paper, it should be possible to refine the timeline of speciation and test the possible explanations based on complete genome sequencing of gorilla and other primates, which is already underway at several centers including the Broad Institute.

    Paper reference: Patterson N et al. (2006). Genetic evidence for complex speciation of humans and chimpanzees. Nature (advance online publication) DOI: 10.1038/nature04789

    See also: here & here

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    Re: Human ancestors and chimps may have dated


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    Sv: Re: Human ancestors and chimps may have dated

    An interesting detail is that many esotericists have been saying this for years.

    The aryosophists sometimes even said that several human races were the results of man-ape-hybridisation.

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    Re: Sv: Re: Human ancestors and chimps may have dated

    Lanz von Liebenfels was one of this esotericists......

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    Re: Human ancestors and chimps may have dated

    If this is true.. Does that mean every human is a hominid-pan hybrid?

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